With the development of mechanical engineering, there has been an increased demand for precision machining techniques for special materials, In particular, as an automobile industry is developed, a demand for precision machining techniques for nonferrous metallic materials, such as aluminum and copper, has been considerably increased.
When an insert or a tool made of WC or HSS is coated with diamond so as to meet the above-described demand, the life span of the insert or tool is improved along with its cutting speed and machining precision. Diamond is the hardest of all materials on Earth, has a relatively great thermal conductivity and does not react with nonferrous metals, so the life span of the insert or tool, its cutting speed and its machining precision are improved when an insert or a tool made of WC or HSS is coated with diamond.
A variety of techniques for growing a diamond thin film at a low pressure have been developed, and may be classified into the following five categories:
1. Thermal Activation or High-temperature Filament Activation Chemical Vapor Deposition (Chemical Vapor Deposition is abridged as “CVD”, hereinafter) (S. Matsumoto, Y. Sato, M. Kamo, N. Sekada, Jpn, J. Appli. Phys.21, part 2, L183 (1982) et al.);
2. High-frequency Plasma Enhanced CVD (M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth 62, 642 (1983); S. Matsumoto and Y. Matsui, J. Mater. Sci., 18, 1785 (1983); S. Matsumoto, J. Mater. Sci. Lett, 4, 600 (1985); S. Matsumoto, M. Hino and T. Kobayashi, Appl. Phys. Lett., 51, 737 (1987));
3. Direct Current Discharge Enhanced CVD (K. Suzuki, A. Sawabe, H. Yasuda and T. Inuzuka, Appl. Phys. Lett., 50, 728 (1987));
4. Combustion Frame Utilizing Method (Y. Hirose and M. Mitsuizumi, New Diamond, 4, 34 (1988)); and
5. Other Mixed Methods.
All these techniques are based upon an art of generating atomic hydrogen in the vicinity of the surface of a growing film. The most generally used synthetic method is Plasma Assisted CVD. In accordance with this method, a diamond film of high quality can be obtained. Many variations of this technique are utilized at a laboratory stage, but are not commercialized yet.
There have been developed a variety of apparatuses that can perform a diamond film depositing method. An apparatus proposed by Kamo et al. has a structure in which a silica tube having a diameter of about 40 mm is extended through a sleeve attached to a wave guide. In this apparatus, the silica tube is utilized as a deposition chamber, and the power (such as 2.45 GHz or 915 MHz) of microwaves is transmitted to the deposition chamber through a set of wave guides, a power monitor and a tuner. The plasma is controlled to be situated in the center portion of the chamber, and a substrate supported by an aluminum basket is situated at the location. A mixture of hydrogen and methane is allowed to pass through the chamber and microwaves are applied to induce the discharge of microwaves, so the deposition of a diamond film is carried out.
Thereafter, a diamond film depositing apparatus using a magnet-microwaves and plasma reactor developed by Kawarada et al. is known.
In an apparatus developed by Bachmann et al., a bell jar is employed as a microwaves and plasma reactor for growing a diamond film (P. K. Bachmann, W. Drawl, D. Knight, R. Weimer and R F. Messier, in Extended Abstracts Diamond and Diamond-like Materials Synthesis, edited by G. Johnson, A. Badzian, and M. Geis, Materials Research Society, Pittsburgh, Pa., 1988, p. 0.99).
U.S. Pat. No. 5,311,103 discloses a newly developed diamond depositing apparatus using microwaves and plasma. As illustrated in FIG. 1, a cavity 12 has an inner diameter of about 178 mm, and is defined by a cylinder 14 open at its top. In the lower portion of the cavity 12 are situated a metallic step 18A, a metallic base plate 20 and a metallic tube 22. The cavity 12 is closed at its bottom by a cavity bottom surface 18 supported on a metallic screen 24 mounted under a bottom plate 25 separated from a base plate 20 by a central ring plate 25A. An excitation probe 30 is held through an interior sleeve 104 by insulators 110 and 110A.
Source gas is supplied through a source gas inflow hole 36 and a source gas ring 38, and fills the lower portion of the cavity 12 by the restriction of a quartz disk chamber 40. The base plate 20 and the quartz disk chamber 40 are cooled by a water cooling channel 42 and a gas cooling channel 44 that communicate with a water cooling ring 42A and a gas cooling ring 44A, respectively.
A substrate 50, on which a diamond film is deposited, is placed on a susceptor 51 supported by a nonmetallic tube 52. The nonmetallic tube 52 is mounted on a movable stage 54 that is used to change the position of the substrate 50 with regard to plasma 56. The movable stage 54 is connected to a movable rod 58 that is extended through a vacuum chamber 60.
The apparatus 10 is installed over a vacuum chamber 62 that is comprised of a chamber tube 62, which is connected to a vacuum pump 68, and a chamber wall 64.
A water cooling line 70 passes through a slidable short 16 and around a metallic cylinder 14. Air cooling passages 72 and 74 are provided to cool the cavity 12.
As described above, U.S. Pat. No. 5,311,103 discloses a diamond film depositing apparatus in which the diffusion of plasma is performed within a space defined by the wall of a bell-shaped jar. Though the volume of plasma is relatively small, the volume is sufficient to cover a deposition area of a diameter of about 200 mm, thus being superior in energy efficiency. This apparatus has the following advantages; a relatively great energy efficiency due to the restriction of plasma diffusion to a relatively small space, the stable and reliable control of plasma diffusion, the relatively easy control of impurities due to the elimination of contact of plasma with a cavity resonator sidewall, and the relatively easy maintenance of a reactor due to the elimination of the necessity for keeping the interior of the cavity resonator vacuous. In order to place and remove a workpiece, the bell jar and the cavity resonator can be disassembled. In tests, it was found that the apparatus is capable of depositing a diamond film on any one of various workpieces under pressure of 0.5 to 100 Torr.
The patented apparatus has been successfully utilized to deposit a diamond film on a variety of workpieces. A conventional cavity resonator has a cylindrical shape that is radially and axially symmetrical.
In the patented apparatus, the power of microwaves is preferably concentrated on a portion from which plasma is diffused. Since the power of plasma and microwaves is concentrated on a workpiece, energy efficiency can be improved and unnecessary heating can be prevented.
However, as will be described in FIG. 4a, when a cylindrically shaped cavity is employed, the power of microwaves is concentrated not only on a portion from which plasma is diffused, but also on the upper and side portions of the cavity 12. Accordingly, the probe 30 and a cavity wall 14 are heated. As a result, in order to secure a working space greater than a certain size, it is necessary to increase the power of the microwaves. When the power is increased, the probe and the cavity wall are heated, thus requiring the cooling of them. Actually, in the apparatus disclosed in U.S. Pat. No. 5,311,103, a water cooling line 70 and air cooling pipes 72 and 74 are employed.